It looks like a new "locally integrated menu" will make it into Unity, starting with version 5.6.0. There's basically no information about the new locally integrated menubar, except for two bug reports which link to some custom Unity, Compiz, Metacity and Light Themes branches so to see "LIM" in action, I've compiled all these branches and here's the result. The "locally integrated menubar" can be displayed on the panel (for maximized windows) as well as in the window decoration (unmaximized windows). But it's not displayed on both in the same time.